yes

Ok quick thory frisk is the knight cus they already know about suzy ralsei and kris everything cus they prob already played the entirerty of deltarune from the monster that talked about it so that means deltarune and undertale are connected through a game deltarune has undertale and undertale has a copy of deltarune and they could probably hop through worlds and frisk is perhaps looking for the soul becaus when the knigh opend their chest it was EMPTY WITH NO SOUL IT WAS JUST A ORB FIRSK IS THE KNIGHT THEY ARE THE KNIGH LOOKING FOR THEIR MISSING SOUL/PLAYER ITS ALL F**KING CONECTED also the reson frisk is looking for the player/soul is becaus the prob dropped it in the deltarune world

And why is frisk opening fountains?
Geno frisk or pacifist frisk?
(If it's pacifist this my explain why they tried to kidnap toriel, but not why they kidnapped undyne)

ok so i have an answer for this. Frisk is opening fountains because they think the soul can only be seen in the dark worlds, so they are looking for the soul in the fountains, because they think its somewhere in there
And for the second question, it's probably neutral frisk or geno, idk abt paci

But the knight works with frisk, they Know about the soul

No the knight is frisk and they are working with Kris because they need to find a good opportunity when Kris can give Frisk the soul, which is not going to their plan

Why didn't they just gave the soul to carol?

prob cus they dont trust Carol enough to take care of an inter-dimensional chuawa